The TDx assay for cyclosporine and its metabolites in blood samples compared with HPLC and RIA methods

Two methodologies have been developed to monitor cyclosporine (CsA) therapy: high-performance liquid chromatography (HPLC) and radioimmunoassay (RIA). Recently, a fluorescence polarization immunoassay (FPIA) has also become commercially available for the assay of CsA and its metabolites. The authors compared the results obtained with a modified FPIA with those found with two RIAs which use a polyclonal antibody, in order to verify if the FPIA assay is suitable for routine measurements in blood samples. Moreover, the accuracy of the RIAs and of the modified FPIA was checked against the results obtained by an HPLC technique assumed as a reference assay. The FPIA assay for CsA in blood samples seems preferable to the RIAs; in fact, as far as specificity is concerned, the TDx assay is comparable to polyclonal RIAs, while the precision (both within- and between-laboratories) is significantly better. Moreover, the TDx method is easier and faster to perform (20 samples can be assayed in about 30 min, while 2-4 h are necessary with RIA), with fewer handling steps; the instrumentation is automated and the reagents are more stable and less hazardous than those used in RIA.